Currently, over twelve million people worldwide suffer from Alzheimer's Disease (AD). This number is predicted to quadruple over the next 40 years. As such, treatment of AD represents a major unmet medical need. Currently approved medicines to treat AD may help ameliorate symptoms, but are not effective to stop progression of the disease.
Without being bound by a particular theory, it is believed that the pathology of Alzheimer's disease (“AD”) involves amyloid-β (“Aβ”) peptides, which are metabolites of β-amyloid precursor protein (Alzheimer's disease-associated precursor protein or “APP”), and are believed to be major pathological determinants of AD. These peptides exist mainly in 40 or 42 amino acid forms, Aβ1-40 (“Aβ40”) and Aβ1-42 (“Aβ42”), respectively. Aβ40 and Aβ42 are generated by two enzymatic cleavages occurring close to the C-terminus of APP, subsequent to cleavage by beta secretase. The enzymes responsible for the cleavage, β-secretase and γ-secretase, generate the N- and C-termini of Aβ, respectively. The amino terminus of Aβ is formed by β-secretase cleavage between methionine residue 596 and aspartate residue 597 of APP (numbering based on APP 695 isoform). γ-secretase cleaves at varying positions 38-, 40- or 42-residues at the C-terminal of this β-secretase cleavage product to release the Aβ peptides. A third enzyme, α-secretase, cleaves the precursor protein between the β- and γ-cleavage sites, thus precluding Aβ production and releasing an approximately 3 kDa peptide known as P3, which is non-pathological. Both β- and α-secretase cleavage also result in soluble, secreted-terminal fragments of APP, known as sAPPβ and sAPPα, respectively. The sAPPα fragment has been suggested to be neuroprotective. For example, γ-secretase also cleaves Notch-1 protein and is believed to have other substrates. Direct acting gamma-secretase inhibitors have substantial and unwanted side effects due to the effects on development pathways requiring Notch cleavage. Little is known about the molecular mechanisms that confer substrate specificity on this potentially promiscuous enzyme. Gamma secretase enzyme is known to contain four subunits: presenilin, nicastrin, anterior pharynx-defective 1 (APH-1), and presenilin enhancer 2 (PEN-2).
In normal individuals, the Aβ peptide is found in two predominant forms, the majority Aβ-40 (also known as Aβ1-40) form and the minority Aβ42 (also known as Aβ1-42) form, each having a distinct COOH-terminus. The major histological lesions of AD are neuritic plaques and neurofibrillary tangles occurring in affected brain regions. Neuritic plaques consist of Aβ peptides, primarily Aβ40 and Aβ42. Although healthy neurons produce at least ten times more Aβ40 compared to Aβ42, plaques contain a larger proportion of the less soluble Aβ42. Patients with the most common form of familial Alzheimer's disease show an increase in the amount of the Aβ42 form. The Aβ40 form is not associated with early deposits of amyloid plaques. In contrast, the Aβ42 form accumulates early and predominantly in the parenchymal plaques and there is strong evidence that Aβ42 plays a major role in amyloid plaque deposits in familial Alzheimer's disease patients. Neurofibrillary tangles consist of aggregated tau protein and their role in AD pathology is less clear. AD symptoms are most closely correlated with total brain Aβ rather than plaques. About 10% of AD cases result from autosomal dominant inheritance of mutations in either the APP or the presenilin 1 and presenilin 2 genes. In both cases, increased production of total Aβ or Aβ42 versus Aβ40 results.
The N2a cell system has been extensively studied as a model system of Aβ production relevant to neurodegeneration in AD. Assays measuring production of Aβ in N2a cells are known, wherein Aβ-production activity is evaluated e.g., by Aβ ELISA assay and/or by Western blotting. Various agents including compounds such as Gleevec (Imatinib, STI571) have previously been shown to be capable of lowering Aβ levels in the N2a cell system at drug concentrations of below 10 μM.
International Patent Publication No. WO 03/057165 discloses that certain previously known inhibitors of tyrosine kinases, such as imatinib, are useful to inhibit the production of and accumulation of Aβ. Netzer et al., Proc Natl Acad Sci., 100(21):12444-9 (2003) showed that imatinib inhibits production of Aβ without affecting γ-secretase cleavage of Notch-1 and without unacceptable toxicity to the neurons. Imatinib is not an ideal drug for treating AD, however, because it does not penetrate the blood brain barrier very well, and it has other biological effects. The specific target of imatinib for inhibition of production of and accumulation of Aβ has not been defined, so finding improved derivatives presents challenges.